1. Field of the Invention
This invention relates to a large diameter hot rolled steel bar having a novel metallurgical structure in cross section, excellent in strength as well as toughness and a process for the production of the same by controllig the perlite transformation temperature.
2. Description of the Prior Art
Up to the present time, hot rolled steel rods have generally been cooled by the so-called Pb patenting using a lead bath for cooling, air patenting or warm water patenting, but these methods have some problems. In the Pb patenting method, for example, rolled steel rods with a higher strength can be obtained, but the use of a lead bath results in worsening of the working environment, i.e. environmental pollution. Moreover, the air patenting or warm water patenting method has the drawback that cooling cannot be effected more stably and rapidly as compared with the Pb patenting and method and accordingly, the phase transformation cannot be accelerated at a low temperature.
On the other hand, high strength steel rods can also be obtained by quenching and tempering, but the so-called tempered martensite steel has a tendency to be inferior to perlite steel in the delayed fracture property. Thus, it is necessary from the standpoint of reliability to obtain a higher strength in perlite steel. However, in the case of a large diameter steel bar, in particular, having a diameter exceeding 20 mm which holds a very large quantity of heat, the cooling rate is slow and the perlite transformation cannot be effected at a low temperature by the air patenting or warm water patenting method of the prior art. That is, the transformation temperature becomes higher than in the case of small diameter rods and it is difficult to obtain a tensile strength of at least 120 kg/mm.sup.2 without addition of large amounts of elements of increasing hardenability.
High carbon steel bars requiring a high strength, such as steel bars for PC (prestressed concrete), have hitherto been produced by heating a billet, hot rolling and then cooling at a certain cooling rate in a cooling bed, for example, by natural air cooling, forced air coolig or mist cooling, thereby causing perlite transformation in the steel of austenite structure.
When the hot rolled steel rod of austenite structure is cooled immediately after rolling to cause perlite transformation, however, there arises a problem in that the toughness is low immediately after the production. As indexes for indicating the toughness, there are values as to elongation and reduction of area and the lowering of toughness corresponds to that of the reduction of area, in particular. Since the toughness, i.e. reduction of area is recovered by ageing after the passage of a long time such as several hundred hours or longer, although not completely, even when the steel rod is naturally allowed to stand, this is not a large disadvantage when producers have generally a lot of the stock and there is a long period of time until the steel bar is used, as in the past. Of late, however, the variety of steels, outer shapes, etc. have been so diversified that producers cannot have a lot of the stock corresponding thereto and cannot avoid putting the stock on the market, depending upon the use, before a complete ageing recovery is attained. In this case, the toughness is not uniform in the steel bar and there is often danger, in particular, when using under a large tension.
Thus, in order to improve the toughness, it is taken into consideration to carry out a forced ageing treatment, but the thus resulting steel bar has also a disadvantage that the yield stress is low in proportion to the breaking stress. Generally, a PC material is loaded with a stress of 70 to 80% to the yield stress of a steel bar, so a higher yield stress is desired for the steel bar. In addition, the above described steel bar has further disadvantages of being inferior in straightness and ease of handling.
Hot rolled steel bars of medium or high carbon steel are ordinarily produced by heating a billet and then rolling at once to a final shape in several to several ten stages, and after rolling, cooling the steel rod in a cooling bed, whereby to transform the austenite structure to perlite structure and to give a relatively high tension steel.
The properties of medium or high carbon steels are varied by the conditions of a heat treatment applied and in the prior art method comprising rolling a billet at once and cooling, the temperature distribution of the billet in a heating furnace affects the temperature distribution of the rolled material during and after the final rolling, thus resulting in dispersion of the mechanical properties of the product in the longitudinal direction. When the rolling operation runs into trouble on the way even if the temperature distribution in a heating furnace is uniform, delay of the rolling line takes place and the temperature of the rolled material is thus lowered, resulting in dispersion of the mechanical properties of the product similarly.